The subtle side to hypoxia inducible factor (HIFalpha) regulation.

The transcription factor hypoxia inducible factor alpha-subunit (HIFalpha) is pivotal in the cellular response to the stress of hypoxia. Post-translational modification of HIFalpha by hydroxylase enzymes has recently been identified as a key "oxygen sensing" mechanism within the cell. The absence of the substrate oxygen prevents the hydroxylases from modifying HIFalpha during hypoxia and allows dramatic up-regulation of both HIFalpha protein stability and transcriptional activation capability. In addition to this oxygen-dependent response, increased HIFalpha protein levels and/or enhanced transcriptional activity during normoxic conditions can be stimulated by various receptor-mediated factors such as growth-factors and cytokines (insulin, insulin-like growth factor 1 or 2, endothelial growth factor, tumour necrosis factor alpha, angiotensin-2). Oncogenes are also capable of HIFalpha activation. This induction is generally less intense than that stimulated by hypoxia and although not fully elucidated, appears to occur via hypoxia-independent, receptor-mediated signal pathways involving either phosphatidyl-inositol-3-kinase/Akt or mitogen activated protein kinase (MAPK) pathways, depending on the cell-type. Activation of Akt increases HIFalpha protein synthesis in the cell and results in increased HIFalpha protein and transcriptional activity. MAPK also activates HIFalpha protein synthesis and additionally may potentiate HIF1alpha transcriptional activity via a separate mechanism that does not necessarily require protein stabilization. Here we review the mechanisms and function of receptor-mediated signals in the multifaceted regulation of HIFalpha.